1. Field of the Invention
This invention relates to a device and method for characterizing the quality of a sheet of paper, and more specifically, to a device and method which measure the distribution of cellulose fibers in a sheet of paper by monitoring the variations in the intensity of a narrow beam of light transmitted through the sheet as the sheet moves perpendicularly through the beam.
2. Description of the Related Art
Paper is produced from a suspension of fibers. These fibers are usually made of cellulose, derived mainly from wood and rags. The evenness of the distribution of these fibers in a sheet of paper is of paramount importance to the optical, mechanical and printing properties of the sheet. Therefore, one of the chief goals for a paper maker is to develop a paper making process and adjust the parameters of the process to achieve as even a "basis weight" or distribution of these fibers in the finished sheet material as possible. In the paper making art, the term "basis weight" refers to the weight of the paper-forming fibers per unit area of the sheet surface. When the fibers are distributed evenly and the paper has a uniform basis weight, the sheet of paper will have its greatest strength, will look and feel smooth, and will be receptive to sharply defined lines of print. Conversely, local variations in the basis weight will yield a sheet having poor strength. This is because stress is concentrated in the areas of the sheet having fewer fibers, so that these areas of the sheet tear first. Furthermore, sheets having uneven basis weight may look and feel rough and will blur printed lines.
To characterize the quality of a sheet of paper, paper makers refer to the "formation" of the sheet. There is, apparently, no standard definition of "formation." However, for the present purpose "formation" will be defined as the manner in which fibers forming a paper sheet are distributed, disposed and intermixed within the sheet. In all paper sheets, the sheet-forming fibers are, at least to a certain extent, unevenly distributed in bunches called "flocs." However, sheets of paper having generally evenly distributed, intertwined fibers are said to have good formation. Conversely, when the fibers forming the sheet are unacceptably unevenly distributed in flocs, the paper sheet is grainy rather than uniform and is said to have poor formation.
A variety of devices exist for measuring various characteristics of the formation of paper sheets. In one such device, called a basis weight sensor (or microdensitometer), a beam of light is transmitted through the sheet as the sheet passes perpendicularly through the beam. The intensity of the beam is measured by a light detector after the beam is transmitted through the paper sheet. This light detector is positioned on the opposite side of the sheet from the light source. The light detector produces an electrical signal indicative of the intensity of the transmitted beam. As the basis weight of the portion of sheet through which the light beam is passing increases, the intensity of the beam transmitted through the sheet decreases. Thus, the electrical signal from the light detector is indicative of the basis weight of the sheet. In this type of basis weight sensor, the "light source" may be a source of visible light or alternatively may be some other type of radiation, as a beta ray device. That is, a variety of source/detector combinations may be used as long as the detector can sense the amount of radiation passing through the paper.
As previously mentioned, the fibers forming every sheet of paper tend to congregate in flocs. In any one sheet, these flocs will have a variety of sizes. Thus, as the paper moves perpendicularly through the light beam, the electrical signal produced by the light detector will be modulated at a plurality of frequencies corresponding to the distribution of floc sizes and also to the speed with which the paper sheet moves through the light beam. As the sheet speed increases, the frequency with which the flocs modulate the electrical basis weight signal increases. Similarly, smaller flocs modulate the signal at higher frequencies than larger flocs. The amplitude of these modulations corresponds to the local variations in basis weight or, what amounts to the same thing, the local variations in the distribution of the fibers forming the flocs.
In one technique, the formation characterizing device displays the average peak-to-peak variation in the electrical signal produced by a basis weight sensor. The average peak-to-peak value of the electrical signal is said to indicate the magnitude of variations in the basis weight of the sheet. However, for the reasons discussed below, this technique may give a false indication of the sheet formation.
In many instances, the paper maker will want to make a sheet having as even a fiber distribution as possible, i.e. one having good formation. To accomplish this, the paper maker will want to know, not only the magnitude of the variations in basis weight, but also the size distribution of the flocs. The paper maker will also want to know the strength of the lowest basis weight portions of the sheet. However, the previously described technique, which yields only the average peak-to-peak value of the basis weight signal, gives no indication of the size of the flocs creating these variations in the basis weight signal or the strength of the weakest areas of the sheet. Thus, this technique fails to completely characterize sheet formation.
In another technique for characterizing sheet formation, a beta radiograph is made of a sample sheet of paper. Light is then passed through or reflected off of the radiograph. Variations in the intensity of a narrow beam of this light are converted into an electrical signal as the radiograph moves, at a uniform speed, perpendicularly with respect to the beam. A graphical display is produced of the amplitude of the modulations of this electrical signal as a function of the wavelengths comprising the signal. This display is called a wavelength power spectrum. FIG. 1 illustrates one such display for several grades of paper having good, intermediate and poor formation. This technique has been discussed in great detail by Norman and Wahren in a number of papers, including their symposium paper "Mass Distribution and Sheet Properties of Paper".
For some commercial paper manufacturing situations, the Norman and Wahren technique may not be appropriate. First, as illustrated in FIG. 1, there is little difference between the wavelength power spectra of a well-formed sheet and a poorly-formed sheet at wavelengths below about 1 mm, whereas a considerable difference can be seen for wavelengths between about one millimeter and thirty-two millimeters. Thus, the Norman and Wahren technique produces more information than may be necessary for the paper maker to determine the formation of the sheet. Second, this technique provides so much information that its interpretation may be difficult for the non-expert: the paper maker may prefer a device and technique that provides him or her with only a few numbers, which together completely characterize the formation of the sheet, rather than an entire spectral display. Third, this technique, like the previously described technique for measuring the average peak-to-peak value of a basis weight signal, fails to provide the paper maker with an indication of the strength of the weakest portions of the sheet. Finally, this technique requires considerable time for the analysis of a given paper sample, and is restricted to "off-line" analysis -- i.e., to samples which have been removed from the paper machine -- whereas the paper maker may prefer to have an "on-line" device capable of rapidly analyzing the formation of the paper as it is actually being produced on the machine.